In our earlier U.S. Pat. No. 5,224,346, we describe experiments which show that the amount of nitrogen oxides (NO and NO.sub.2), or NOx in engine exhaust gases can be reduced by injecting ammonia (NH.sub.3), which reacts with nitrogen oxides to produce nitrogen and water. As described in that patent, it is desirable to control the amount of ammonia injected so it is generally proportional to the production of NOx. Ammonia injection generally is not required when a warmed-up engine is idling or the like, when the relatively small amount of NOx can be eliminated by the hot catalytic converter. At other times, the amount of NOx produced, and consequently the amount of ammonia to be injected, is indicated by the amplitude of many phenomena. These include the exhaust temperature, the air inlet manifold pressure, and the flow rate of fuel or air into the engine. Special sensors could be installed in an engine and connected to control circuitry that controls ammonia injection, but the installation of such sensors increases the cost for an ammonia injection system. A control for an ammonia injection system which avoided the need for installation of a special sensor, would greatly reduce the cost of an ammonia injection system.
In the above-mentioned U.S. Pat. No. 5,224,346, applicant described the injection of ammonia through a small tube having an opening pointed downstream along the exhaust gas flow. Although such injector tube lay near the upper end of the exhaust gas conduit where exhaust gases are very hot, the exhaust gases cool quickly and the ammonia was not rapidly mixed with the exhaust gases. As a result, a large part of the ammonia did not effectively combine with nitrogen oxides in the exhaust gases. An ammonia injection system which increased the amount of ammonia that reacted with nitrogen oxides in the exhaust gases, would reduce the amount of ammonia required and decrease the amount of nitrogen oxides emitted into the atmosphere.